1. Field of the Invention
This invention relates generally to solder joints, and more particularly to a method and composition for improving the strength of lead/tin solder joints without sacrificing wetability.
2. Prior Art
One very popular low melting composition for soldering joints is a lead/tin composition which nominally contains about 95% lead and 5% tin. One particular application for this material is reverse chip bonding of integrated circuit chips to ceramic substrates. The principal reason for the presence of the tin in this composition is to enhance the wetability of the solder with respect to the surface or surfaces to which the solder is being applied. With amounts of tin less than about 4% or 5%, the solder does not effectively wet the surface which in many cases contributes to poor solder joints, the failure mechanism normally being the failure of the solder to bond to the surface because it did not properly wet. However, the presence of tin in the solder composition does have certain drawbacks, the principle of these being that the solder joint is somewhat more brittle than a solder joint of pure lead or very low alloy content lead. Thus, the use of 95/5 lead/tin solder is a trade-off, the tin being present in just enough amount to provide the desired wetability of the underlying surface, but in no greater amount than is necessary since even this small amount does significantly degrade the ductility of the joint. Hence, in this trade-off, in the soldering with lead solders one provides just enough tin to insure proper wetting of the surface but only this minimal amount to minimize the degradation of the quality of the solder joints. One prior art technique for reducing tin that has been proposed is found in U.S. Pat. No. 4,332,343 in which carboxylic acids are introduced into the flux to react with the tin in the solder. This has not proven to be entirely effective. Also, certain other additives to flux compositions for other purposes have been proposed. For example, very small amounts (0.002% to 0.005%) of lead carbonate are added to fluxes to prevent biodegradation during storage are taught by U.S. Pat. No. 4,360,392; and catalysts for causing certain reactions within soldering fluxes are disclosed in U.S. Pat. No. 4,165,244, which catalysts include a wide range of organo-metallic's. U.S. Pat. No. 3,970,239 teaches reacting lead oxide with an acetate compound, which will form a coordinate lead compound.